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Gas flow hydrodynamics in an oxygen delignification retention tower Pineault, Isabelle

Abstract

Gas flow hydrodynamics were characterized as a function of pulp suspension concentration in a laboratory-scale residence tower. Three kraft pulps were studied: never-dried brown stock (kappa 33), a fully bleached kraft pulp (88-89% brightness) that never had been dried, and a dried and reslurried fully bleached kraft pulp (88-89% brightness). The following experiments were performed to charaterize gas flow in pulp retention towers. First, gas flow in a narrow rectangular tower was characterised as a function of suspension mass concentration by calculating the bubble rising velocity. These results were compared with bubble rising velocity measurement made in a pilot-scale column having a L/D ratio of 6.5:1 (inside diameter of27.94 cm).. The same column was used to measure gas holdup with both the gas disengagement technique [Daly et al., 1992] and measurements of the density variation method along the column height. Finally, mass transfer was evaluated in a small laboratory-scale column ( L/D = 4.1:1) using a dissolved oxygen probe. The main results of these experiments are as follows. First is was found that at medium pulp consistency, there are no bubbles formed in the tower due to the lack o f free water. Second, at pulp consistencies higher than 5 %, gas is trapped in the fibre network, which increases considerably the gas holdup. Furthermore, this gas holdup is not influenced by the gas flow rate. Third, the bubble size at low consistencies is influenced by the air flow rate as well as the pulp consistency present in the column. Fourth, the type of pulp used did not have a great influence on the gas holdup in the pilot-scale column. Finally, mass transfer occurs in the laboratory-scale column, with kL a values in the same range as the those reported in bubble columns.

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